Process for preparing rare earth oxide coated phosphors

ABSTRACT

The present invention provides surface coated phosphors useful in field emission displays and vacuum fluorescent displays. The surface coated phosphor comprises a thin coating of rare earth oxide, e.g., yttrium oxide, disposed on an uncoated phosphor such as a sulfide phosphor. The present invention further provides a process for preparing a surface coated phosphor comprising contacting the uncoated phosphor with a rare earth hydroxide gel solution to obtain a rare earth hydroxide gel coated phosphor, drying the gel coated phosphor to remove solvent residues, and heat treating the dried rare earth hydroxide gel coated phosphor. The surface coated phosphors of the present invention have improved cathodoluminescence efficiency, coulombic aging resistance, chemical, and/or oxidative stability.

FIELD OF THE INVENTION

[0001] The present invention generally relates to rare earth oxidecoated phosphors useful in field emission display devices and vacuumfluorescent display devices and a process for preparing the coatedphosphors, and particularly, to rare earth oxide coated sulfidephosphors having improved cathodoluminescence properties and a processfor preparing them.

BACKGROUND OF THE INVENTION

[0002] The operating characteristics of field emission displays (FED)and vacuum fluorescent displays (VFD) place many demands uponcathodoluminescent phosphors. In particular, the lower operatingvoltages used in FEDs compared to cathode ray tubes (CRT) result insmaller penetration depths and reduced luminous efficiency andbrightness. This reduction in luminous efficiency can be compensated forby using higher drive currents or current densities and taking advantageof the longer pixel dwell time (e.g., 30 μs) used in FEDs.

[0003] However, the use of higher current densities acceleratescoulombic aging, e.g., degradation by electron bombardment, as well asdegradation of the phosphor by interactions with the residual atmospherein the vacuum devices. The phosphor degradation can lead to poisoning ofthe field emission cathodes. Further, since low voltages are employed,operation of the device becomes especially challenging in view of theresistance produced by contaminations such as the surface dead layer.The foregoing shows that there exists a need for low voltage phosphorshaving both high efficiency and chemical stability.

[0004] Furthermore, the phosphors undergo a baking process during themanufacture of VFDs and FEDs during which the surface of the phosphor isoxidized to some extent. The oxidation causes deterioration of thecrystallinity of the phosphor surface, and consequently, the oxidizedsurface decomposes or degrades during operation of the devices. Inaddition, certain phosphors, e.g., as sulfide phosphors, undergo anelectron stimulated decomposition which generates gases such as gasescontaining S, SO, and/or SO₂. These gases are harmful, and they canreduce the electron emission efficiency of the cathode. Thus, therefurther exists a need for phosphors, particularly sulfide phosphors,that have increased resistance to oxidative and/or electron stimulateddecomposition.

[0005] These and other advantages of the present invention, as well asadditional inventive features, will become apparent from the descriptionof the present invention provided herein.

BRIEF SUMMARY OF THE INVENTION

[0006] Many of the foregoing needs have been fulfilled by the presentinvention which provides a surface coated phosphor comprising a thincoating of a rare earth oxide disposed on an uncoated phosphor. Thepresent invention further provides a process for preparing the coatedphosphor comprising contacting the uncoated phosphor with a rare earthhydroxide gel solution to obtain a rare earth hydroxide gel coatedphosphor and heat treating the rare earth hydroxide gel coated phosphor.

[0007] While the invention has been described and disclosed below inconnection with certain embodiments and procedures, it is not intendedto limit the invention to those specific embodiments. Rather it isintended to cover all such alternative embodiments and modifications asfall within the spirit and scope of the invention.

BRIEF DESCRIPTION OF THE DRAWING

[0008]FIG. 1 depicts the relative cathodoluminescence efficiency as afunction of aging time under a pulsed electron beam for an uncoatedZnS:Cu phosphor and the phosphor coated with Y₂O₃ in accordance with anembodiment of the present invention.

DETAILED DESCRIPTION OF EMBODIMENTS

[0009] The present invention provides a surface coated phosphor havingone or more advantages such as improved cathodoluminescence efficiency,improved aging resistance, oxidative stability, and chemical stability.The surface coated phosphor of the present invention find use in FEDsand VFDs. The surface coated phosphors of the present invention combinethe advantageous chemical and/or oxidative stability of rare earthoxides and the luminescent properties of the phosphors. The rare earthoxide coating protects the surface of the phosphor against chemical andoxidative degradation. The rare earth oxide coating also passivates thesurface of the phosphor, for example, prevents formation ofrecombination centers.

[0010] The rare earth oxides have large electron penetration depths,e.g., Y₂O₃ has an estimated penetration depth of 12 nm under a 2 kVaccelerating voltage. As the phosphors, particularly sulfide phosphors,also have large penetration depths, e.g., ZnS has a penetration depth of35 nm under a 2 kV accelerating voltage, the rare earth oxide coatedphosphors are attractive for use in FEDs and VFDs. The surface coatedphosphors of the present invention do not easily degrade or releaseharmful gases such as S, SO, and/or SO₂.

[0011] The surface coated phosphor of the present invention comprises athin coating of rare earth oxide disposed on an uncoated phosphor. Theuncoated phosphor can be any suitable phosphor, for example, a sulfideor an oxide phosphor. An example of a sulfide phosphor is a zinc sulfidebased phosphor such as ZnS:Cu; ZnS:Cu,Al; and (Zn,Cd)S:Ag,Al, orcombinations thereof.

[0012] The surface coating can be made of any suitable rare earth oxide,e.g., an oxide of yttrium, scandium, of a lanthanide, or combinationsthereof. Examples of rare earth oxides include Gd₂O₃, Yb₂O₃, andpreferably Y₂O₃.

[0013] The uncoated phosphor can be of any suitable shape. For example,the uncoated phosphor can be a powder having a particle of size of fromabout 0.01 μm to about 5 μm or larger. The coating of rare earth oxidecan have any suitable thickness, typically greater than 1 nm, preferablyfrom about 10 nm to about 1000 nm, and in some embodiments greater thanabout 1000 nm.

[0014] The present invention provides a process for preparing a surfacecoated phosphor comprising a thin coating of a rare earth oxide disposedon an uncoated phosphor, the process comprising contacting the uncoatedphosphor with a rare earth hydroxide gel solution to obtain a rare earthhydroxide gel coated phosphor, drying the rare earth hydroxide gelcoated phosphor to obtain a dried rare earth hydroxide gel coatedphosphor, and heat treating the dried rare earth hydroxide gel coatedphosphor. In a preferred embodiment, the present invention provides aprocess for preparing a surface coated phosphor comprising a thincoating of rare earth oxide disposed on an uncoated phosphor comprising:

[0015] (a) preparing a solution of a rare earth alkoxide in a mediumcomprising an organic solvent and water;

[0016] (b) heating the solution from (a) to hydrolyze the rare earthalkoxide to obtain a solution containing rare earth hydroxide gel;

[0017] (c) contacting the uncoated phosphor with the solution obtainedin (b) to obtain a gel coated phosphor;

[0018] (d) drying the gel coated phosphor; and

[0019] (e) heat treating or firing the dried phosphor obtained in (d).

[0020] The rare earth hydroxide solution can be prepared by any suitablemethod, for example, by dissolving a precursor of the rare earth oxidein a medium comprising an organic solvent followed by the addition ofwater to the solution. Alternatively, the precursor can be dissolved ina medium containing an organic solvent and water.

[0021] Any suitable precursor, for example, a rare earth compound,preferably a rare earth organic compound which eventually provides arare earth oxide, can be used. It is further preferred that any residueformed from the rare earth compound is easily removable, e.g., byevaporation or oxidation, from the rare earth oxide. A preferred organiccompound is rare earth alkoxide. Any suitable rare earth alkoxide can beused, preferably an alkoxide that provides high hydrolysis rates to formthe hydroxide gel, for example, C₁-C₈ alkoxides, preferably C₁-C₄alkoxides, and more preferably C₂-C₄ alkoxides. In certain embodimentsof the present invention, an isopropoxide is used. Yttrium isopropoxideis an example of a preferred precursor.

[0022] The dissolution medium can include any suitable solvent ormixture of solvents. For example a mixture of polar and nonpolarsolvents can be employed. Suitable nonpolar solvents includehydrocarbons, e.g., aromatic hydrocarbons including toluene. Suitablepolar solvents include alcohols, e.g., ethanol, isopropanol, or butanol,and ketones, e.g., acetone and methyl ethyl ketone. The mediumpreferably includes a mixture of toluene and an alcohol or ketone.Preferred solvent mixtures include toluene and a polar solvent such asisopropanol or acetone.

[0023] The solvent mixture can have any suitable proportion of nonpolarand polar solvents. For example, the nonpolar solvent and the polarsolvent can be present in a volume ratio of from about 20:80 to about80:20, preferably from about 40:60 to about 60:40, and more preferablyfrom about 45:55 to about 55:45. In accordance with certain embodimentsof the present invention, the nonpolar solvent and the polar solvent canbe present in a volume ratio of 50:50.

[0024] The precursor can be dissolved in the medium in an amount toprovide the desired rare earth oxide concentration. The concentration ofthe precursor dissolved is typically greater than 0.1% by weight, e.g.,from about 0.5% to about 5.0% by weight, preferably from about 0.5% toabout 2.0% by weight, and more preferably about 1.0% by weight of thesolution.

[0025] The rare earth metal present in the precursor solution is thentreated so that the metal is ionized and hydrated. This can beaccomplished by hydrolyzing the precursor. Hydrolysis of the precursorcan be carried out by methods known to those skilled in the art, forexample, by contacting the precursor solution with water or an aqueousbase. For example, water or a base can be added to the precursorsolution and heated, preferably with vigorously stirring of thesolution.

[0026] Examples of suitable bases include ammonia or ammonium hydroxideand urea. As urea releases ammonia when heated in aqueous medium, itprovides a convenient way of supplying ammonia to the solution. Theaddition of urea reduces the formation of momentary regions of high pHthat may be encountered in direct addition of a base. High pH conditionsare preferably avoided since they may cause homogeneous nucleation ofthe hydrated rare earth cation.

[0027] To carry out the hydrolysis reaction, water is preferably used inan amount that is in excess relative to the rare earth oxide. Thus, forexample, the molar ratio of water to the precursor can be greater thanabout 10:1, preferably greater than about 100:1, and more preferablyfrom about 100:1 to about 300:1.

[0028] The hydrolysis reaction can be accelerated by heating theprecursor solution. For example, the precursor solution can be heated ata temperature of from about 40° C. to about 100° C., and preferably fromabout 50° C. to about 80° C. In certain embodiments, the solution isheated at the solvent reflux temperature. Heating is carried out untilthe hydrolysis is substantially or preferably fully completed. As therate of hydrolysis increases with temperature, the heating period willdepend upon the temperature. The higher the temperature, the shorter theheating time. The solution can be heated for a period of greater thanabout 0.1 hour, e.g., for a period of from about 1 hour to about 72hours, preferably from about 10 hour to about 30 hours, and morepreferably for about 20 hours, at the above temperature ranges, andpreferably at 65° C.

[0029] The pH of the solution containing the rare earth hydroxide playsan important role in the quality of the coating ultimately obtained.Particularly, it has been found that a heterogeneous nucleation of therare earth hydroxide is desirable to obtain a thin, smooth andcontinuous coating, and that such a nucleation can be achieved bykeeping the hydroxide solution, for example, at a pH of from about 4.0to about 10.0, preferably from about 6.0 to about 8.0 and morepreferably at about 7.5. At low pH conditions, e.g., below about 4.0,the hydrolysis rate of the alkoxide tends to be low. The coatingthickness also tends to diminish under these low pH conditions.

[0030] After the hydrolysis reaction is completed, the phosphor iscontacted with the rare earth hydroxide gel solution. The contacting canbe carried out by stirring the phosphor particles into the solution. Thegel coated particles are separated from the gel solution, e.g., byfiltration or decantation.

[0031] The gel coated phosphor particles are first dried to remove theadsorbed solvents. For example, the particles can be dried at theambient temperature (22±3° C.) or at a moderately higher temperature.Thus, the drying can be carried out at a temperature, e.g., at about 30°C. or higher, such as from about 60° C. to about 150° C., and preferablyat a temperature of from about 80° C. to about 120° C. The drying can becarried in a suitable atmosphere, e.g., in air, vacuum, or in thepresence of a gas such as an inert gas.

[0032] The dried gel coated phosphor particles are then heat treated orfired at a higher temperature. This heat treatment increases the bondingof the gel to the phosphor particles. The temperature at which the heattreatment is carried out can be about 200° C. or higher, e.g., fromabout 225° C. to about 500° C., preferably from about 250° C. to about450° C., and more preferably from about 300° C. to about 400° C.

[0033] The heat treatment can be carried out in a suitable atmosphere,e.g., in air, vacuum, or in the presence of a gas such as an inert gas.Heat treatment in an oxidizing atmosphere, e.g., in air is preferredsince under these conditions, any carbon residues are burned off. Inaccordance with certain embodiments of the present invention, at thehigher temperatures, e.g., about 300° C., the bonding between the geland particle increases. At higher temperatures, the gel undergoescertain physical or morphological changes such as densification and/orcrystallization of the gel into an oxide.

[0034] The surface coated phosphors of the present invention are free orsubstantially free of bridging or agglomeration between the particles.If desired, the coating can be further refined by suitablepost-treatment. For example, any crack in the coating can be healed orany broken bridge area can be rounded off by increasing the heattreatment temperature. Up to the termination of the of crystallizationrange, heat treatment causes the coatings to become more uniform.

[0035] The surface coated phosphors of the present invention haveimproved cathodoluminescence (CL) efficiency relative to the uncoatedphosphor. For example, the CL efficiency is higher than the uncoatedphosphor up to about 10% or more at 1 kV. The surface coated phosphorsof the present invention show improved stability to the intense electronbombardment as well as chemical and oxidative stability. The protectivecoating reduces the thickness of the surface dead layer.

[0036] The following illustrative examples further illustrate thepresent invention but, of course, should not be construed as in any waylimiting its scope.

EXAMPLE 1

[0037] This Example illustrates a method of preparing an yttrium oxidecoated phosphor in accordance with an embodiment of the presentinvention.

[0038] Yttrium isopropoxide was first dissolved in a 50:50 by volumemixture of toluene/isopropanol solution. Water was then added withmixing to the isopropoxide solution to achieve a water-to-yttriumisopropoxide molar ratio of 200:1. The resulting solution was refluxedat 65° C. for 20 hours to completely hydrolyze the isopropoxide. Theamount of yttrium isopropoxide was chosen so as to provide ultimately aY₂O₃ coating of 1.0% by weight of the phosphor. The pH of the solutionwas 7.5. ZnS:Cu phosphor particles were added to the solution, stirred,and the coated phosphor particles were isolated and dried in air. Thecoated phosphor particles were then fired in air at 400° C. for 1 hour.The resulting coated phosphor particles had a continuous, smooth, anduniform coverage.

EXAMPLE 2

[0039] This Example illustrates an advantage of the coated phosphor inaccordance with an embodiment of the present invention.

[0040] The coated phosphor particles prepared as in Example 1 weretested for cathodoluminescence (CL) properties. Coated and uncoatedphosphor screens were made with a screen weight of 2 mg/m2. As shown inFIG. 1, the Y₂O₃ coated phosphor exhibited an improved aging behaviorcompared to the uncoated phosphor. After 60,000 seconds of aging under apulsed electron beam, the coated phosphor has a CL efficiency of about10% higher than the uncoated phosphor.

[0041] While this invention has been described with emphasis upon apreferred embodiment, it will be obvious to those of ordinary skill inthe art that the described product or process may be varied. It isintended that the invention may be practiced otherwise than asspecifically described herein. Accordingly, this invention includes allmodifications encompassed within the spirit and scope of the followingclaims.

What is claimed is:
 1. A surface coated phosphor comprising a thincoating of a rare earth oxide disposed on an uncoated phosphor.
 2. Thesurface coated phosphor of claim 1, wherein said uncoated phosphor is asulfide or oxide phosphor.
 3. The surface coated phosphor of claim 2,wherein said uncoated phosphor is a sulfide phosphor.
 4. The surfacecoated phosphor of claim 3, wherein said sulfide phosphor is a ZnS basedphosphor.
 5. The surface coated phosphor of claim 4, wherein said ZnSbased phosphor is selected from the group consisting of ZnS:Cu;ZnS:Cu,Al; (Zn,Cd)S:Ag,Al; and combinations thereof.
 6. The surfacecoated phosphor of claim 5, wherein said ZnS based phosphor is ZnS:Cu.7. The surface coated phosphor of claim 1, wherein said rare earth oxideis Y₂O₃.
 8. A process for preparing a surface coated phosphor comprisinga thin coating of a rare earth oxide disposed on an uncoated phosphor,the process comprising contacting said uncoated phosphor with a rareearth hydroxide gel solution to obtain a rare earth hydroxide gel coatedphosphor, drying said rare earth hydroxide gel coated phosphor to obtaina dried rare earth hydroxide gel coated phosphor, and heat treating saiddried rare earth hydroxide gel coated phosphor.
 9. The process of claim8, wherein said rare earth hydroxide gel solution is prepared bydissolving a precursor of said rare earth oxide in a medium comprisingan organic solvent to obtain a precursor solution, optionally addingwater to the precursor solution, and further optionally heating theprecursor solution.
 10. The process of claim 9, wherein the precursor isa rare earth organic compound.
 11. The process of claim 10, wherein therare earth organic compound is a rare earth alkoxide.
 12. The process ofclaim 11, wherein said rare earth alkoxide is a rare earth isopropoxide.13. The process of claim 9, wherein said organic solvent is toluene. 14.The process of claim 8, wherein said heat treating of the dried rareearth hydroxide gel coated phosphor is carried out at a temperature offrom about 225° C. to about 500° C.
 15. The process of claim 8, whereinsaid uncoated phosphor is a sulfide or oxide phosphor.
 16. The processof claim 8, wherein said uncoated phosphor is a sulfide phosphor. 17.The process of claim 16, wherein said sulfide phosphor is a ZnS basedphosphor.
 18. The process of claim 17, wherein said ZnS based phosphoris ZnS:Cu.
 19. The process of claim 8, wherein said rare earth oxide isY₂O₃.
 20. The rare earth oxide coated phosphor prepared by the processof claim
 8. 21. A process for preparing a surface coated phosphorcomprising a thin coating of rare earth oxide disposed on an uncoatedphosphor comprising: (a) preparing a solution of a rare earth alkoxidein a medium comprising an organic solvent and water; (b) heating thesolution from (a) to hydrolyze the rare earth alkoxide to obtain asolution containing rare earth hydroxide gel; (c) contacting theuncoated phosphor with the solution obtained in (b) to obtain a gelcoated phosphor; (d) drying the gel coated phosphor; and (e) heattreating the dried phosphor obtained in (d).